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Recent variability in the hyrdological cycle of tropical Asia from oxygen isotopes of tree celulose

RECENT VARIABILITY IN THE HYDROLOGICAL CYCLE OF TROPICAL ASIA FROM OXYGEN ISOTOPES OF TREE CELLULOSE
by
Mengfan Zhu
________________________________________________________________________
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
August 2012
Copyright 2012 Mengfan Zhu

This dissertation investigates hydrological variability within tropical Asia over the past several few centuries as reflected in the stable oxygen isotope composition of atmospheric moisture. The stable isotopes of water in the climate system are unique tracers of moisture transport and tropical rainfall variability. The isotopic signal of atmospheric moisture within the tropics is transferred to cellulose of tropical trees during photosynthesis. Thus, the isotopic composition of tree cellulose can provide an archive of past hydrologic variability through isotopic reconstructions of the cellulose extracted from annual rings of long lived trees. The tropical atmospheric variability reflected in tropical trees can include variations in the Indian Monsoon and changes in moisture availability over Asia in response to the El Niño-Southern Oscillation (ENSO). Here an attempt has been made to better understand how the atmospheric dynamics associated within these climate phenomena influence the isotopic composition of tree cellulose and how these climate signatures have changed through time. High-resolution water isotope records are developed from trees collected from northern Thailand, southern Cambodia, and eastern part of the Tibetan Plateau. These records are examined to assess whether and how the 20th century is unique in terms of the hydrological conditions in tropical Asia under the influences of both monsoon and ENSO with the observed temperature changes. ❧ In northern Thailand, the oxygen isotopic composition (δ¹⁸O) of tree cellulose samples of Pinus kesiya from a montane forest has been analyzed in subannual resolution for the past 80 years. The cellulose δ¹⁸O values exhibit a distinctive annual cycle with an amplitude of up to 12‰, which is interpreted to reflect primarily the seasonal cycle of precipitation δ¹⁸O. The cellulose δ¹⁸O annual mean values correlate significantly with the amount of summer monsoon precipitation over the India subcontinent, corroborating recent studies that suggest the so-called “isotope amount effect” in the tropical precipitation δ¹⁸O reflects the hydrological processes of the upstream or the moisture source regions instead of the rainfall amount at the local site. No obvious trend in the summer monsoon precipitation is detected from the cellulose δ¹⁸O record. However, the record does suggest a temporal weakening relationship between the Indian Monsoon and ENSO over the 20th century. The annual maxima in the cellulose δ¹⁸O values are representative of the moisture balance during the winter dry season, and possibly document a decreasing trend in the isotopically-distinct fog water input during the dry season because of the warming in the 20th century. ❧ Isotope chronologies of Pinus merkusii from a coastal lowland forest in Cambodia have been generated to investigate hydrological variability over the Indo-Pacific Warm Pool (IPWP), based on the aforementioned observation that the δ¹⁸O in precipitation reflects the hydroclimate of the moisture source region. The IPWP is a major source of heat and moisture to the atmosphere and thus strongly influences the global climate. Recognizing its past variability is crucial for understanding large scale climate dynamics such as ENSO. The subannual cellulose δ¹⁸O has been replicated with multiple tree cores which span the past 140 years. The analysis of model outputs from a water isotope-enabled atmospheric model is used in conjunction with isotopic data from tree cellulose and precipitation to investigate moisture sources and how these sources of moisture vary on different timescales. The cellulose δ¹⁸O exhibits strong correlations with convection intensity and precipitation amount over the IPWP. Spectral analysis of the cellulose δ¹⁸O reveals significant peaks at 2-7 years corresponding to ENSO frequencies. This cellulose δ¹⁸O record exhibits no clear overall trend, but the period of 1880s through 1910s is characterized by relatively enriched cellulose δ¹⁸O values, which possibly indicates a background condition in the tropical Pacific Ocean that was more El Niño-like. ❧ Stable isotope dendrochronologies have been developed for sites across the eastern Tibetan Plateau. The plateau plays a pivotal role in creating the land-sea thermal contrast that is theorized to drive the summer monsoon circulation. Today, precipitation across the southern Tibetan Plateau is dominated by moisture carried by the summer monsoon winds. The isotopic composition of this moisture varies with the precipitation amount. Over the northern Tibetan Plateau, atmospheric moisture that falls as precipitation is from westerly air masses that carry a large fraction of recycled moisture across the continent. The isotopic composition of this moisture source is distinct from monsoon moisture over the southern Plateau. In addition, there is an important temperature effect that affects the isotopic fractionation of oxygen isotopes in precipitation over the northern and central Plateau. An inter-comparison of annually resolved tree cellulose δ¹⁸O values from two sites on the Tibetan Plateau forming a latitudinal transect, together with data from a former study are used here to investigate the interplay of these different moisture sources and temperature influence over time. The data from the northerly site indicate the late 20th century was the warmest and/or driest period of the past 500 years in the northeastern Tibetan Plateau, whereas evidence from the southerly site suggests there has been an increase in the strength of the summer monsoon circulation since the 1940s compared to the past two centuries. ❧ These studies have demonstrated that tree cellulose δ¹⁸O is a robust high-resolution proxy for hydroclimate in the past. In tropical Southeast Asia, tree cellulose δ¹⁸O could shed light on surface hydrology of the IPWP that is closely related to ENSO. Observations from tree cellulose δ¹⁸O in my studies have suggested no overall trend in the frequency or magnitude of ENSO variability through the 20th century, but a possible El Niño-like condition from the 1880s to the 1910s. Tree cellulose δ¹⁸O in northern Thailand indicates no overall trend in the monsoon precipitation in South and Southeast Asia. However, the new isotopic data presented here suggest monsoon circulation has intensified in the late 20th century across the eastern Tibetan Plateau.

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RECENT VARIABILITY IN THE HYDROLOGICAL CYCLE OF TROPICAL ASIA FROM OXYGEN ISOTOPES OF TREE CELLULOSE
by
Mengfan Zhu
________________________________________________________________________
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
August 2012
Copyright 2012 Mengfan Zhu